Conventionally, a nitrile rubber (acrylonitrile-butadiene copolymer rubber) has been used as a material for an automotive rubber part such as a hose or a tube by taking advantage of fuel oil resistance, a mechanical characteristic, chemical resistance, and the like. In addition, a hydrogenated nitrile rubber (highly saturated nitrile rubber) obtained by hydrogenating a carbon-carbon double bond in a polymer main chain of a nitrile rubber has better heat resistance, and has been used for a rubber part such as a hose, a sealing material, a gasket, or a diaphragm.
Recently, characteristics required for an automotive rubber part have become severe. Particularly, a rubber part used for a sealing material needs to have excellent original state properties (tensile strength, elongation, and tensile stress), heat aging resistance, fuel oil resistance, compression set resistance, and the like.
For such a situation, a rubber composition obtained by using a carboxyl group-containing highly saturated nitrile rubber containing 1 to 60% by weight of α,β-ethylenically unsaturated dicarboxylic acid monoester monomer units and a highly saturated nitrile rubber containing 0.9% by weight or less of α,β-ethylenically unsaturated dicarboxylic acid monoester monomer units at a specific ratio, and blending a polyamide resin thereto has been proposed (Patent Literature 1). By using this rubber composition, a cross-linked rubber product having excellent roll processability in addition to excellent original state properties and fuel oil resistance can be obtained. However, for example, in a cross-linked rubber product carrying a space like an O-ring, compression set tends to be increased due to a force applied during compression. Therefore, further improvement in compression set resistance has been desired.